1. Field of the Invention
The present invention relates to an electrostatic transfer type liquid electrophotographic printer, and more particularly, to an electrostatic transfer type liquid electrophotographic printer preventing images from deteriorating due to an increase of a liquid carrier on a transfer belt when the images are transferred from the photosensitive drums and overlapped onto the transfer belt.
2. Description of the Related Art
In general, an electrophotographic printer, such as a laser printer, is an image forming apparatus in which a latent electrostatic image is formed on a photosensitive medium, such as a photosensitive drum or photosensitive belt. The latent electrostatic image is developed using toner with a predetermined color, the developed image is transferred onto paper, and thereby a desired image is obtained.
The electrophotographic printer is classified into a liquid electrophotographic printer and a dry-type electrophotographic printer according to the kind of toner. Ink in which toner particles are mixed with a liquid carrier to a predetermined concentration is used in the liquid electrophotographic printer. The liquid electrophotographic printer using the liquid ink has a printing quality better than that of the dry-type electrophotographic printer using toner having powder particles and can easily form a color image.
FIG. 1 is a schematic diagram of a conventional electrostatic transfer type liquid electrophotographic printer. Referring to FIG. 1, the electrostatic transfer type liquid electrophotographic printer includes image forming units 10 developing an image with a predetermined color on photosensitive drums 12, a transfer belt 20 onto which the developed color image is transferred, a first transfer roller 30, and a second transfer roller 40 transferring the transferred color image on paper P.
The transfer belt 20 rotates to contact a roller 22, a transfer backup roller 24, and guide rollers 26 and 28.
The image forming unit 10 includes a main charger 14 charging a surface of the photosensitive drum 12 to a predetermined potential, a laser scanning unit 16 projecting light onto the surface of the charged photosensitive drum 12 and forming a latent electrostatic image, and a developing unit 18 developing the latent electrostatic image to a toner image using ink with a predetermined color.
In order to transfer the toner image developed on the surface of the photosensitive drum 12 onto the transfer belt 20, the first transfer roller 30 to which a voltage having an opposite polarity to that of the toner is applied, is installed. This transfer belt 20 moves between the first transfer roller 30 and the photosensitive drum 12. And this first transfer roller 30 applies a predetermined pressure to the photosensitive drum 12 and the transfer belt 20.
Four sets of the image forming unit 10 and the first transfer roller 30 corresponding to the image forming unit 10 are provided and are arranged in series in a traveling direction of the transfer belt 20 so that images with colors such as yellow (Y), magenta (M), cyan (C), and black (K), are sequentially formed.
The second transfer roller 40 is installed to correspond to the transfer backup roller 24, and a voltage having a polarity opposite to that of the toner is supplied to the second transfer roller 40 so that color images formed on the transfer belt 20 are transferred by an electrostatic force onto the paper P transferred between the transfer backup roller 24 and the second transfer roller 40. This second transfer roller 40 is disposed in a downstream of the traveling direction of the transfer belt 20 from the image forming units 10 and the first transfer backup rollers 30.
In general, a concentration of the toner image formed on a first photosensitive drum 12 when the developed color images are sequentially transferred onto the transfer belt 20 from the photosensitive drums 12, is about 20%, and the liquid carrier is partially squeezed when about 4 kgf of pressure is applied to the transfer belt 20, and thereby the concentration of the toner image is increased slightly. Subsequently, when the toner image on a second photosensitive drum 12 is transferred and overlapped onto the transfer belt 20, the concentration of the toner image at an overlapped and transferred portion is maintained about 20%. However, as an amount of the liquid carrier per area is increased during transferring the image, the images may be pulled out (dragged), and edges of the images may be spread out (blotted) and overlapped. When the transferring of the image is repeatedly performed, the transferred images may severely deteriorate.